More and more retirees are being taxed on Social Security benefits. Legislative changes over the decade have shaped the age of receipt, amount of benefits and taxation over the decades, meaning many Americans unexpectedly will face taxation on the public program paid into during their working years.

This deduction would likely benefit people who earn modest incomes. The lowest-earning seniors already pay no federal income taxes, so they wouldn’t benefit. The highest earners make too much to qualify.

A new bill offers seniors a $4,000 tax deduction, partially fulfilling Trump’s pledge to end Social Security taxes. But the limited scope and temporary nature of the relief reveal the challenge of broader tax reform.